Dissolved phosphate decreases the stability of amorphous ferric arsenate and nano-crystalline yukonite

Extensive research has been conducted on the competitive adsorption of arsenate (AsO₄^3-) and phosphate (PO₄^3-) to mineral surfaces, but the stability of ferric arsenate mineral(oid)s under elevated phosphate levels remains poorly understood. Therefore, we investigated the impact of dissolved phosphate (0, 0.5, 50 mM) on the stability of amorphous ferric arsenate (AFA; FeAsO₄·nH₂O) and nano-crystalline yukonite [Ca₂Fe₃(AsO₄)₃(OH)₄·4H₂O], both synthetic and contained in natural As-contaminated soil (∼16 g/kg As) and mine-waste material (∼39 g/kg As) for up to one year. Substantial amounts of As (∼45% of total As) were released into solution from AFA and yukonite at high phosphate concentrations due to incongruent dissolution of the solids and substitution of arsenate by phosphate in both mineral(oids). After one year, both solids sequestered ∼8 wt% P with approximately 20-30% accounting for adsorbed and precipitated species. This P increase was also observed in the soil and mine-waste samples, where AFA and yukonite comprised up to 4.3 and 4.9 wt% P, respectively. The high reactivity of ferric arsenates with aqueous phosphate may lead to a substantial overestimation of adsorbed As determined by sequential As extractions of materials containing these phases and requires increased caution when applying phosphate to stabilize polymetallic mine wastes. Furthermore, long-term phosphate additions via fertilization of As-contaminated soil or renaturalized mine tailings containing amorphous or nano-crystalline ferric arsenates should be reduced to limit the export of As(V) into surface streams and groundwater.